Quicktime File Format (2012-08-14).pdf

Profile Atom GuidelinesUniversal Featuresfeature-valueUnsigned int(32) indicating maximum audio bit rate in bits per secondFeature ValuesThe value is an unsigned 32-bit integer indicating the maximum audio bit rate in bits per second that mustbe supported to guarantee playback of the audio. The actual maximum bit rate may be smaller, so a readershould not display this as the current bit rate.Example: 128 kbps = 128000.Writer ResponsibilitiesA writer of the maximum audio bit rate feature should record a value that is equal to or greater than the currentbit rate for the sound track. While the value may exceed the actual maximum bit-rate, the writer should attemptto minimize any over-estimation.While recording the precise bit rate is preferred, it is not required. A writer (such as a CE device) may chooseinstead to record a constant value so long as that value is greater than or equal to the bit rate that may beencoded. It is also permitted to record a value set by the audio encoder during initialization so long as thevalue is never exceeded.Feature Value AlgorithmApple recommends a sliding average over 1 second calculated from the sample tables.If the feature is written for a newly encoded track (as by a CE device), it is permitted to record a value used toinitialize the audio encoder so long as the value is never exceeded.If the sound track is edited, and the audio bit rate is not constant, the maximum audio bit rate must berecalculated. Note that editing can change the duration of media samples, resulting in non-constant bit rateaudio even when the sound track is encoded using a constant bit rate encoder. Maximum bit rate may becalculated as a sliding average over 1 second, based on the sample table. This can be calculated in the followingmanner:1. For each sample, calculate the average 1-second bit rate; choose the shortest run of samples, includingthe candidate sample, that comprise 1 second or more of audio, then divide the total data size of thosesamples by their total duration.2. Choose the maximum value from the list of calculated 1-second averages.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.402